Assessing the Interaction in Maize Cropping Density, Nitrogen and Soil Moisture with a Systems Simulator in Semi-Arid Machakos District, Kenya

ABSTRACT

The APSIM model was used to evaluate a range of maize densities on a sandy-clay loam at Kenya Agricultural Research Institute (KARJ)-Katumani Research Centre in Machakos and 2 contrasting farmers' fields situated on sandy and clay soil types. Each simulation was initialised with actual soil parameters measured at the time of commencing the run. Simulations on the Katumani soil were done under 5 contrasting Short Rainy i SR.) seasons of different rainfall regimes, namely SR1996 (185 mm), SR1995 (270 mm), SR1991 (328 mm), SR1992 (720 mm), and the El Nino season of SR1997 (958 mm). Maize densities used were 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 plants/m². The model showed that maize grain yield declined as density increased under the poor 1996 season. Optimum crop density increased with seasonal rainfall, to reach 5.0 plants/m² in the 1992 and 1997 heavy rainfall seasons. On the farmers' fields, APSIM showed that the clay soil out-yielded the sandy soil in high rainfall, while the reverse was true on poorer seasons. Simulations were done to determine the effects of varying nitrogen at constant crop density of 5.3 plants/m². Nitrogen rates used were 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, and 120 kg/ha. The response of maize to N fertiliser varied with seasonal rainfall. Grain yields were depressed when N was added to maize during the poorer SR1996 and SR1995 seasons. With higher seasonal rainfall, APSIM predicted a sharp increase in grain yield at low rates of N. Simulations of the effects of N on maize yield on the 3 farmers' fields (clay and sandy soils) were done for SR1997 (1310 mm) and SR1998 (122 mm) seasons, using N rates of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 kg ha. The model predicted crop failure for the SR1998 season on the clay soil throughout all levels of N rates. In the wet SR1997, grain yield was low at 0 to 30 kg N/ha, and then increased sharply at levels greater than 30 kg N/ha. The response to N was, therefore, highest in the clay soil under wet conditions. Grain yield responded well in the sandy soil during SR1997, though the rate of response was lower than in the clay soil. However, the sandy soil proved superior to clay in the poor season (SR1998). A gradual response to N was predicted in the sanity soil, up to 30 kg N/ha. Sowing date was more important for sandy than clay soil type. Delayed sowing on the clay soil type did not adversely affect yield, whereas a week's delay on the sandy soils led to a yield decline even in a favourable season.